Currently, arrays of small disk drives are used as main storage facilities for mainframe host processors. Such small disk drives employ resident microprocessors which restrict reads and writes to fixed block/sector sizes. Such drives are further configured to handle data records and/or logical tracks that are spread across plural drives (with parity) to enable data recovery in the event of the malfunction of one or more disk drives.
Host processors generally employ a variable length record configuration when communicating with disk memory. For instance, many host processors configure their data in the form of count, key and data fields, with the count field being of a fixed length and the key and data fields being of variable length. To write a variable length field into a fixed length sector on a disk drive requires that the microprocessor associated with the disk drive segment the variable length record into fixed length sector sizes. Further, provision is made to enable a variable length record to start or end at other than a sector break, to enable an efficient use of disk storage space.
For efficient transfers of data to occur between a host processor and a plurality of small disk drives, it is preferable that each disk drive look to the host processor as though it is employing variable length records, and that the host processor look to each disk drive as though it is employing fixed length sector formats. In accomplishing such a transparency, a number of further requirements are placed upon the interface circuitry. For instance, accomplishment of transparency must not exact a substantial penalty in data throughput rate; must enable efficient use of the disk space; must enable accommodation of altered record sizes, and preferably, be able to extract small amounts of data from large blocks of data (e.g., full track).
Accordingly, it is an object of this invention to provide an improved method for transfer of variable length records from a host processor to a disk drive which stores data in fixed length sectors.
It is another object of this invention to provide a method of data transfer which enables compressed data to be handled and accommodated in a disk drive's fixed length sectors.
It is yet another object of this invention to provide a method of data transfer which enables a variable length record input data stream to be reblocked for storage in fixed length sectors on disk, using a minimum amount of disk capacity.
It is still another object of this invention to enable a disk, having fixed length sectors, to transfer data from any point in a track, without requiring such a transfer to begin at the commencement of the track.